The present invention is a result of the proliferation of low power, Spread Spectrum radio modem devices in the 902-928 MHz, 2.4 GHz and 5.7 GHz bands. Popularity of these radio devices is largely due to FCC regulations that allow appropriately certified radio transceivers to be operated license free. This certification requirement restricts the transmitter output power in order to enable many users to share the band. Further, since the radios are Spread Spectrum devices, they can generally tolerate interference from other radios transmitting in the same geographical area.
Many of these prior art devices were designed and intended for short range operation (less than 1000 feet, for example, due to the low transmit power restrictions and the requirement of unobstructed line-of- site between antennas for maximum range). However, if external outdoor gain antennas are placed on tall buildings or radio towers, considerable line-of-site ranges (measured in miles) are possible. The problem here is that the losses in the typical, inexpensive coaxial transmission line between the radio and the antenna at these frequencies can be excessive unless prohibitively expensive cable is used. Putting an antenna on a tall radio tower or building would give clear line of sight to many locations, but this is largely defeated by the transmission cable loss.
In a typical RF bi-directional amplifier application, a duplex amplifier with heavy filtering, such as in U.S. Pat. No. 5,502,715 to Penny, is used. However, this is in general unsatisfactory due to the fact that not only are both transmit and receive amplifiers are on at all times, thus leading to wasteful power usage, but also heavy filtering is also necessary to keep the transmit and receive signals from interfering with each other, leading to further expense and power wastage. Still further, since each transmit and receive signal must be put to a separate frequency to avoid interference, this design is wasteful of spectrum bandwidth, a scarce commodity in many applications.